The present invention relates to corrosion protection. In particular, it relates to corrosion protection of mechanical parts used in the manufacture of computer disk drives.
A computer hard disk drive typically has at least one disk having a magnetic recording surface, and a magnetic transducer for reading information from the disk and writing information onto the disk. When the disk drive is in operation, an air cushion forms between the disk and the transducer. Due to the air cushion, the transducer flies at a distance called "flying height" by computer component manufacturers. The flying height in a high performance disk drive is typically about 5 microinches to about 15 microinches (1,270-3,810 angstroms) above a spinning hard disk.
If any particulates come between the disk and transducer, the transducer may become aerodynamically unstable. This instability eventually leads to catastrophic head crash.
Repair of crashed hard drives at this time is not economical. A hard disk drive that has experienced a head crash must typically be replaced. Thus, it is essential to address all potential sources of contamination in a hard disk drive. For this reason, hard disk drive components are manufactured and assembled in a very clean environment, referred to as a "white room environment" to workers skilled in the art.
The materials used to construct individual parts of a hard disk drive may also contaminate the drive and cause head crash. Some metals used in forming mechanical parts corrode when exposed to the atmosphere. The corrosion generates particulates such as powder and flakes which break off and interfere with disk drive performance.
Anticorrosive coatings have been applied to the metal parts to inhibit corrosion. These coatings can flake or chip off and contaminate the disk drive. Coatings that are applied to parts to improve wear resistance and increase lubricity, for example, can also ultimately cause component failure if the coating flakes off.
It is essential to select materials in forming individual disk drive components which do not eventually contaminate the assembled hard disk drive.
High performance hard disk drives typically have a number of recording disks, and a number of actuator arms for supporting a number of magnetic read/write transducers near the disk surfaces. All of the actuator arms in the disk drive are supported by a single actuator arm carriage.
The carriage rapidly moves each actuator arm in unison during disk drive operation. Adequate mechanical performance of the carriage is heavily dependent upon the material selected to construct the part. The carriage must be constructed of a material that is rigid, is very light in weight, and has a high modulus of elasticity. Carriages known in the art are built of aluminum metal alloys, and magnesium metal alloys.
Aluminum alloys such as Alloy 356 (sand cast alloy) and Alloy 360 and 380 (die cast alloys) each have a suitable modulus of elasticity, (i.e.--between about 10.3 million and 10.5 million pounds per square inch) but have densities which provide excessive mass to the carriage/actuator arm assembly. Magnesium alloys such as Alloy AZ91B (die cast material), Alloy AZ91C (sand cast material) and AZ31B (wrought material) have a slightly lower modulus of elasticity (i.e.--about 6.5 million pounds per square inch) and are much less dense. A material having a modulus of elasticity of about 6.5 million p.s.i. is still suitable for forming a carriage.
It is estimated that carriages built of the magnesium alloys mentioned above have a mass which is about 65 percent of the weight of a 360 aluminum alloy, for example. A weight reduction of this magnitude allows the carriage/actuator assembly to accelerate much more rapidly, and provides more rapid operation of the hard disk drive. From the standpoint of its mechanical characteristics, the most preferred material for constructing actuator arm carriages is magnesium alloy AZ91B. Alloy AZ91B contains aluminum in an amount of about 9 percent by weight, zinc in an amount of about 1 percent by weight, and manganese in an amount of about 0.2 percent by weight. The balance is magnesium.
Although magnesium alloy AZ91B exhibits superior mechanical characteristics, it is highly reactive with atmospheric constituents. Atmospheric constituents such as oxygen, moisture and chlorine are known to react with the alloy. In particular, AZ91B alloy rapidly corrodes in the presence of oxygen and moisture. Magnesium also reacts with chlorine present in the air, forming magnesium chloride. Corrosion products, such as magnesium oxide and magnesium chloride, for example, flake off and form particulates which are known to cause head crashes.
In order to take advantage of the superior mechanical characteristics of magnesium alloys, it is necessary to treat the surface of the carriage with an anticorrosive coating to prevent the formation of corrosion products. Various anticorrosive coatings have been applied to the exterior surfaces of the carriage. Presently, a Parylene-C coating is used as a corrosion inhibiter for alloy AZ91B. Parylene-C is available from the Nova Tran Corporation of Clear Lake, Wis. Parylene-C coating has the following chemical structure: ##STR1## Although Parylene-C coating is known to initially inhibit corrosion, it has recently been discovered that in as little as one year after coating, severe corrosion of the exterior surfaces of the carriage is present. Anticorrosive coatings must be effective for the life of the disk drive.
Parylene-C has a chlorine atom attached to the aromatic ring which reacts with the magnesium in the alloy, causing corrosion. The primary substance which causes hard disk drive contamination in Parylene-C coated magnesium alloy carriages is magnesium chloride.
If corrosion of any type is discovered before the disk drive component containing the carriage is shipped to a customer, the disk drive component must be disassembled, and the carriage recoated. This procedure is undesirable because the labor costs involved are high, and recoating the carriage is expensive and time consuming. If the disk drive has already been shipped, or put into use, corrosion on the surfaces of the carriage is even more undesirable.
An adequate anticorrosive coating should be thick enough to prevent corrosion, but thin enough to avoid interfering with the conductive properties of the substrate. At thicknesses between about 1/2 mil and about 1 mil (127,000 to 254,000 Angstroms), Parylene-C coatings failed to prevent corrosion in a period of time of less than one year. Because the useful life of a disk drive often exceeds one year, Parylene-C coating has proved to provide an unacceptable corrosion barrier.
Parylene-C coatings at thicknesses of between about 1/2 mil to about 1 mil (127,000-254,000 angstroms) are also known to interfere with the conductive properties of the metal. At thicknesses within this range, the magnesium alloy substrate is less electrically conductive, and is not as capable of dissipating static as the substrate would be without the coating. Components which are not static dissipative are known to arc when subjected to a static charge. Arcing is known to damage the electrical components of the disk drive. The loss of conductivity also causes the substrate to attract particulates such as dust, which cause hard drive crashes.
Other organic coatings have been applied by various methods, such as electrostatic dipping for example, but have produced coatings which are generally too thick, and which vary greatly in thickness across the exterior surfaces of the part. These coatings also are of a thickness which interferes with the electrical properties of the substrate. For example, E-Coating, an epoxy product available from the Glidden Paint, Architectural Maintenance Company, Dublin, Ohio, is applied to carriages by an electrostatic method. E-Coating is generally too thick, and provides a coating which does not have a uniform thickness. Coatings which are too thick or which vary in thickness interfere with the close tolerances required in manufacturing the carriage. Typically the machined portions of the carriage require tolerances of plus or minus 0.003 inch.
Actuator arm carriages are formed by several methods. The carriages may be die cast, sand cast, or machined from wrought alloy. Die casting is the most desirable because the outer surfaces of the part are smoother, and require less machining to manufacture the finished part. Machining a part from wrought alloy is the least desirable because the procedure is time consuming and expensive. Sand casting is an acceptable method, but produces a part having exterior surfaces which are more rough than surfaces formed from a die cast method. The rougher surfaces require more machining, and are more costly to manufacture.
Machining magnesium metal, or magnesium metal alloys, requires an oxygen-free environment to avoid explosions. Also, because the metal is very hard, machining the carriages requires the use of very hard cutting tools.
After machining, the carriages are inspected for signs of corrosion, cleaned and coated with an anticorrosive coating. None of the anticorrosive coatings known in the art adequately prevent corrosion of the magnesium alloy carriage. None of the existing coatings are thin enough to avoid interfering with the close machine tolerances required in the manufacture of the part. None of the existing coatings are thin enough to avoid significantly diminishing the electrostatic properties of the metal. The prior art coatings also do not provide corrosion protection for the entire expected life of the hard disk drive.